The present application relates to instruments and particularly to probes utilizing an elongated body that is inserted into tubular members or openings, for example, a tube inspection instrument which must be inserted into long tubes to inspect the interior of the tubes. Similarly, in the petroleum industry, numerous logging instruments are lowered into boreholes to take measurements of the formation surrounding the borehole. In all of these cases, it is necessary to center the instrument within the tube or the borehole or other opening to obtain accurate measurements.
In the past, the traditional method for centralizing an inspection tool within a tubular member or opening has been the use of bow springs placed around the circumference of the tool. The bow springs exert a force on the tube wall or the borehole wall that centers the tool within the opening. Bow springs are used since they easily deform when the tool passes an obstruction or a portion of the tube or borehole having a reduced diameter. Although the use of centralizing springs is satisfactory in a large number of instances, it does have a particular disadvantage, when used in boreholes, of allowing the instrument to assume an off center position. This occurs when the borehole is inclined and the weight of the instrument tends to deform the springs, which are high friction devices, on the low side of the borehole, and thus position the instrument in an off-center position. In many cases, the off-center position of the logging instrument does not affect the results obtained. In other cases it is desirable, and at times necessary, to ensure that the tool is accurately centered within the tube or the borehole opening.
In an attempt to provide a better means for centering the instrument in the opening, deformable wheels or rollers have been placed on the circumference of the instrument. The use of deformable rollers is possible in the case of tube inspection devices where the tubes have a uniform diameter and only a slight deformation of the rollers is required to allow the instrument to pass any obstruction that is encountered. This type of approach, of course, cannot be used in logging instruments which are lowered into boreholes since the changes in the diameter of the borehole are too great to be accommodated by mere deformable rollers. Thus, the problem exists of providing a means for accurately centering the logging tool in a borehole even in those cases where the borehole diameter changes rapidly. The problem is further complicated in those cases where it is necessary to lower the logging tool through a production tubing string and then allow it to exit out the bottom of the production tubing string into the borehole casing. In this case, the change in diameter can be a factor of 2 to 4 times which greatly exceeds the range that can be accommodated with deformable rollers. Accordingly, the present invention is directed to overcoming these and other problems experienced by the prior art.